The present invention relates to apparatus for producing and guiding a light beam, and relates in particular to a laser and to a sensing head which may be used, for example, in a laser interferometer to measure displacement of an object.
The use of laser interferometer apparatus for measuring displacement is well known. Typically, a laser beam is split into two parts, one of which is directed along a fixed reference path, and the other of which is directed towards an object whose displacement is to be measured. Reflected beams from the fixed reference path and the object are recombined to generate an interference beam whose instantaneous intensity varies with displacement of the object being measured. By monitoring a cyclic variation in the intensity of the interference beam, an incremental count corresponding to displacement of the object relative to some reference position, or datum, may be generated.
It is known to employ such apparatus in order to determine the displacement of a stage which is movable in two mutually orthogonal directions, known as an XY stage.
In such apparatus two laser beams are required to enable two laser interferometers to be used to measure displacements of the stage in two orthogonal directions.
In one apparatus, currently marketed by Hewlett-Packard Corporation, it is known to derive the two laser beams from a single beam output by a laser, by placing one or more beamsplitters in the path of the laser beam to direct different portions of the beam to different parts of the apparatus.
An object of the present invention is to provide a laser which outputs two beams directly.
It is known however from U.S. Pat. No. 5,648,848 to provide a laser comprising a laser tube for generating a laser beam, and a cavity external to the laser tube which receives, at an input end thereof, the beam from the laser, tube wherein two fibre optic cables are provided each of which has an end attached to an output face of the cavity, and an optical system is positioned in the cavity to receive the laser beam, said optical system comprising a birefringent prism which produces from the laser beam two beam parts which are directed into said ends of the fibre optic cables.
One problem associated with such a laser is that the angle of deviation of a laser beam passing through the optical system can vary with changes in temperature.
According to a novel aspect of the invention, the optical system in the cavity comprises two further beam steering prisms which direct the beam parts from the birefringent prism onto the ends of the fibre optic cables.
By this means any deviations in the angles of the beam parts produced by the birefringent prism which are caused by temperature variations in the laser cavity, can be removed by the beam steering prisms which are subjected to the same temperature variations. It is preferable that the three prisms are identical.
The laser is preferably a gas laser which includes a thermally controlled laser tube to which the cavity is attached.
In a preferred embodiment of the invention, the laser tube produces a laser beam which has two orthogonal polarisation states, the birefringent prism splits the laser beam into its two polarisation states and the fibre optic cables which receive the two beam parts are made from polarisation maintaining fibres.